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For the launch screen, I think there may be an Apple bug with xibs and
system colors. In my custom project, when the deployment target was 13,
the launch screen supported dark mode fine, but when I switched to 12,
it didn't any more.

To fix this, I instead used our named color "background_color" in the
xib. This sidesteps the issue because named colors are supported on
both iOS 12 and 13 and it's actually a better experience than having
the background be solid black. This color doesn't show up in Interface
Builder in Xcode because the color isn't defined in an xcassets
collection, but it does work.

The other changes are changing the first-run experience to also use
this background color instead of systemBackgroundColor/black. The
ChromeSigninViewController remains black by default because it is also
presented from settings, which uses the standard Apple color palette.

Bug: 997170
Change-Id: Icd71fa2aedf1f3828fa2cefb5382deacc964cae3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795905
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695597}